The metastable phase equilibrium of the quaternary system LiCl + KCl + RbCl + H2O at 348.15 K was done by using an isothermal evaporation method. The solubilities, densities, and refractive indices of the equilibrated solution were determined experimentally. On the basis of the measured data, the stereo phase diagram, the metastable phase diagram, the water content diagram, and the physicochemical properties versus composition diagrams were constructed. Results show that the quaternary system is of a complex type with the solid solution of potassium chloride and rubidium chloride [(K, Rb)Cl] formed at 348.15 K. The metastable phase diagram consists of two invariant points, five univariant curves, and four crystallization fields corresponding to single salts potassium chloride (KCl), rubidium chloride (RbCl), lithium chloride monohydrate (LiCl center dot H2O), and a solid solution of potassium and rubidium chloride [(K, Rb)Cl]. Salt RbCl has the largest solubility among the coexisting salts and presents the smallest crystallization field in the phase diagram. The crystallization region of [(K, Rb)Cl] almost occupies the entire phase region, which shows that it potassium from rubidium in chloride solution by only using evaporation and crystallization methods. This is compared with the metastable phase diagram of the quaternary system at 298.15 K that shows that the crystallization forms have not changed, while the crystallization fields have slightly change. The refractive index of the solution has been calculated using an empirical equation. The maximum absolute deviation between the calculated and experimental data of the refractive index is less than 0.007.